The Journal of Neurophysiology Vol. 85 No. 4 April 2001, pp. 1522-1532
Copyright ©2001 by the American Physiological Society

Responses and Afferent Pathways of Superficial and Deeper C₁-C₂ Spinal Cells to Intrapericardial Algogenic Chemicals in Rats

 ¹Department of Physiology and  ²Department of Cell Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73190

Qin, Chao, Margaret J. Chandler, Kenneth E. Miller, and Robert D. Foreman. Responses and Afferent Pathways of Superficial and Deeper C₁-C₂ Spinal Cells to Intrapericardial Algogenic Chemicals in Rats. J. Neurophysiol. 85: 1522-1532, 2001. Electrical stimulation of vagal afferents or cardiopulmonary sympathetic afferent fibers excites C₁-C₂ spinal neurons. The purposes of this study were to compare the responses of superficial (depth <0.35 mm) and deeper C₁-C₂ spinal neurons to noxious chemical stimulation of cardiac afferents and determine the relative contribution of vagal and sympathetic afferent pathways for transmission of noxious cardiac afferent input to C₁-C₂ neurons. Extracellular potentials of single C₁-C₂ neurons were recorded in pentobarbital anesthetized and paralyzed male rats. A catheter was placed in the pericardial sac to administer a mixture of algogenic chemicals (0.2 ml) that contained adenosine (10³ M), bradykinin, histamine, serotonin, and prostaglandin E₂ (10⁵ M each). Intrapericardial chemicals changed the activity of 20/106 (19%) C₁-C₂ spinal neurons in the superficial laminae, whereas 76/147 (52%) deeper neurons responded to cardiac noxious input (P < 0.01). Of 96 neurons responsive to cardiac inputs, 48 (50%) were excited (E), 41 (43%) were inhibited (I), and 7 were excited/inhibited (E-I) by intrapericardial chemicals. E or I neurons responsive to intrapericardial chemicals were subdivided into two groups: short-lasting (SL) and long-lasting (LL) response patterns. In superficial gray matter, excitatory responses to cardiac inputs were more likely to be LL-E than SL-E neurons. Mechanical stimulation of the somatic field from the head, neck, and shoulder areas excited 85 of 95 (89%) C₁-C₂ spinal neurons that responded to intrapericardial chemicals; 31 neurons were classified as wide dynamic range, 49 were high threshold, 5 responded only to joint movement, and no neuron was classified as low threshold. For superficial neurons, 53% had small somatic fields and 21% had bilateral fields. In contrast, 31% of the deeper neurons had small somatic fields and 46% had bilateral fields. Ipsilateral cervical vagotomy interrupted cardiac noxious input to 8/30 (6 E, 2 I) neurons; sequential transection of the contralateral cervical vagus nerve (bilateral vagotomy) eliminated the responses to intrapericardial chemicals in 4/22 (3 E, 1 I) neurons. Spinal transection at C₆-C₇ segments to interrupt effects of sympathetic afferent input abolished responses to cardiac input in 10/10 (7 E, 3 I) neurons that still responded after bilateral vagotomy. Results of this study support the concept that C₁-C₂ superficial and deeper spinal neurons play a role in integrating cardiac noxious inputs that travel in both the cervical vagal and/or thoracic sympathetic afferent nerves.